This invention relates to an optical heterodyne interference measuring apparatus and method used, for example, for the alignment or the like of a mask and a wafer.
In prior art exposing apparatuses for the manufacture of semiconductors, an optical heterodyne method as shown in Japanese Laid-Open Patent Application No. 62-58628 has been proposed as a method of highly accurately aligning a mask and a wafer.
Referring to FIG. 27 which shows the construction of an example of the prior art, a beam splitter is provided on the optical path of a two-frequency linearly polarized laser beam source 1, and a condensing lens 3 and a photoelectric detector 4 are successively arranged in the direction of reflection of the beam splitter 2. Also, a polarizing beam splitter 6 is provided on the optical path in the direction of transmission of the beam splitter 2 with a mirror interposed therein. A mirror 7 is provided in the direction of transmission of the polarizing beam splitter 6 and a mirror 8 is provided in the direction of reflection of the polarizing beam splitter 6 so that respective light beams may enter a transmission type diffraction grating 11 on a mask 10 disposed on the underside of a mask stage 9 and a reflection type diffraction grating 14 on a wafer 13 disposed on the upper surface of a wafer stage 12.
Further, a mirror 15 is provided vertically above the mask 10 and wafer 13, and a condensing lens 16 and a photoelectric detector 17 are arranged in the direction of reflection of the mirror 15. The outputs of the photoelectric detectors 4 and 17 are connected to a signal processing control unit 18, the output of which is in turn connected to the mask stage 9 and wafer stage 12.
A light beam emitted from the two-frequency linearly polarized laser beam source 1 is divided into two beams by the beam splitter 2, and one of the two light beams is detected by the photoelectric detector 4 through the condensing lens 3 and is input as a reference beat signal to the signal processing control unit 18. The other light beam enters the polarizing beam splitter 6 via the mirror 5, and is divided into two linearly polarized light beams having only a horizontal component or only a vertical component and slightly differing in frequency from each other. The divided two light beams enter the transmission type diffraction grating 11 at a predetermined angle of incidence via the mirrors 7 and 8, respectively. The diffracted light transmission-diffracted by the transmission type diffraction grating 11 and reflection-diffracted by the reflection type diffraction grating 14 is detected by the photoelectric detector 17 via the mirror 15 and the condensing lens 16, and is input as a diffracted light beat signal to the signal processing control unit 18. The signal processing control unit 18 detects the phase difference between the reference beat signal and the diffracted light beat signal, and drives the mask stage 9 and the wafer stage 12 so that the phase difference may become null, whereby the precise alignment of the mask 10 and wafer 13 is effected.